Production and acid treatment of hydrous chromic oxide



Patented June 6, 1944- UNITED STATES PATENT OFFICE.

PRODUCTION AND ACID TREATMENT F HYDROUS CHROMIC OXIDE Marc Darrin,Baltimore, Md., assignor to Mutual Chemical Company of America, NewYork, N. Y., a corporation of New Jersey,

No Drawing. Original application June 7, 1938,

Serial No. 212,392.

Divided and this application May 9, 1940, Serial No. 334,283

4 Claims.

This invention relates to improvements in the manufacture of chromicoxide pigments, etc.. including a new fiuffy chromic oxide having theempirical formula CrzOs; and methods of manufacturing such pigments.

The invention also includes new intermediate products of suchmanufacture, including almiform hydrated or hydrous chromic oxide, and anew acid combination of the hydrous oxide both in the form of a syrupand in the form of a' uses which will be evident to those skilled in therespective arts in which such pigments are used. It possesses permanenceand resistance to light,

' heat and weathering which is well known for the ordinary denseanhydrous chromic oxide pi ments. It can be readily manufactured in aneconomical manner from available materials, as will more fully appearfrom the following description.

The ordinary method of producing chromic oxide is by firing a mixture ofa bichromate, such as NazCnO'r, with a reducing agent such as sulfur,whereby most of the bichromate is reduced to an insoluble green oxideheld in a sintered mass of sodium sulfate, (Nazism), which is removed byleaching with water, leaving as a residue the ordi- The new fiufiychromic oxide has a distinctive and characteristic structure. Itdisintegrates without grinding to a fine, uniform, porous, soft, fluffygreen powder, the particles of which have a very fine skeleton orsponge-like structure. It

is a valuable paint pigment, having a low apparent specific gravity .anddesirable oil absorption properties, imparting body and viscosity to theoil, without undesirable display of shortness; and

a paste made with oil can be diluted without packing of the pigmentwhich settles as a voluminous soft layer.

- The new fluffy chromic oxide has valuable properties which adapt itfor use as a permanent rpigment, as for example in heat resistant paintsfor hot water and steam pipes, locomotive boilers, furnaces; insun-proof and weather-resistant paints for agricultural machines,fences, roofs, blinds, and the like; in oil, water, lime and mortarcolors, printing inks, colored paper, wallpaper printing, bank notes,plastics, ceramics, roofing granules, glass, porcelain enamels, and1vatrious chemical reactions both direct and catay 1c. i

The new fluify chromic oxide has valuable and distinctive oil absorptionproperties. For example, it combines in a superior manner with paintoils, forms stronger and clearer colors, has a greater range of shades,and possesses greater flexibility of combination with other materials,

. centimeter of "the sifted and lightly packed dry nary dense chromicoxide, having the empirical formula CmOa. For pigment purposes thecoarse and hard crystalline particles are reduced to a fine powder bygrinding, but the product is still very dense and somewhat abrasive incharacter. Its crystal structure has been investigated by Davey (Phys.Review 21, '116'(1923)), by obtaining X-ray diffraction patterns, whoconcludesthat the molecule is hexahedral, consisting of three atoms ofoxygen at the corners of an equilateral triangle with two chromiumatomsimmediately above and below the centre of the triangle, three suchmolecules forming a unit prism. Other investigators indicate similarconclusions. The structural formula may be written as follows:

The true speeme' gravity (A. s. T. M. mas-27) of known grades of purechromic oxide, CmOa (as reported in Gardner Physical, and ChemicalExamination of Paints, -Varnishes, Lacquers, Colors" 7th ed., Oct. 1935,Pigment Index Supplement p. Al-1344) is between 5.04 and 5.20. Theapparent specific gravity (grams per cubic chromic oxide fall between0.300 and 0.900, and

I all fall between 0.200 and 1.200. No known-grades than the ordinarydense forms of chromic oxide.

and tinting strength (A. S. T. M. 15387-36). By

way of comparison the new fiuify oxide responds to mass color andtinting strength tests in a superior manner; the color is a purer andbrighter shade of green both in mass and tint, the hue;

/ can be varied over a greater range between blue- 50.0. No known gradesof pure chromic oxide of which I am aware fall within these limits.

In general the oil absorption characteristics of known grades of purechromic oxide are very poor. Practically no body or viscosity isimparted to the oil, and there is a very decidedly undesirable displayof shortness, such as is well known to those familiar with testing anduse of paint pigments. When thinned the pigment tends to settle andpacks into a dense mass occupying a volume proportionate to its apparentspecific gravity. Likewise it responds unfavorably to running tests,brushing tests and other practical application tests which are de- Themore compendent on proper afilnity between the pigment and oil. In fact,known grades of pure chromic oxide, as far as I am aware, do not havemany of the desired properties of a paint pigment, except green colorand permanence, and would not be employed if other suitable, permanentgreen pigments were available; It is common I toavoid its use, exceptwhen conditions are such that chromic oxide must be used because of itsunique permanence.

The new fiuffy chromic oxide eliminates or greatly reduces the aboveundesired properties, through greater oil absorption, better affinitytowards the oil, very superior body and viscosity, freedom fromshortness, and superior response towards settling tests, running tests,-brushing tests and related practical application tests.

The new ilufiy oxide can be made in stronger and brighter shades, andover a greater range of color. It is better blending, and adapted toincreased fields of applications. pigments it may be thinned withoutgreat loss of body, retaining its shade and hiding power.

This is accomplished without loss of permanency toward light, heat,weathering, lime, etc.

The apparent specific gravity of the new pigment may be less than halfthat-of the ordinary oxide, being in one of its preferred forms between0.300 and 0.900. By methods which will appear from the following itsrange ,can be extended. Its oilabsorption .is above, 18. A verysatisfactory range can be made between 20 and 30. On high dilution ofan'oil paste, the new soft fiufly chromic oxide settles as a voluminoussoft layer which does not pack. The volume is proportionate to itsapparent specific gravity. In physical form it is-a very soft, porousuniformly fine powder, requiring no grinding. The particles have a veryfine sponge-like, or spider-like, skeleton structure.

, Although the'new soft fiuffy chromic oxide is very permanent towardslight and ordinary high temperature, it is not stable at unusual, high.temperatures. On being brought to equi- Like other good librium bycontinued heating at 2300" F. It changes to a dense form similar to-theordinary oxide. Impurities such as alkalie's greatly hasten andavoidance of prolonged heating at such temperature, and also isbenefited by the elimination or removal of impurities from the hydrousoxide before firing which, 'unless removed, would hasten the speed orchange to the ordinary dense form of oxide. Substances which retard thespeed of the final change or hasten the speed of formation of thedesired intermediate product without hastening the final change, arebeneficial. ,In carrying out the firing, it is advantageous that thematerial pass quickly through the lower oxidation temperatures and thenbe held for a limited and regulated time at the temperature at which thedesired change and formation of,the intermediate oxide occurs. Thisvaries somewhat with the presence or absence of other materials, and themethod of firing. In general and by way ofexample, I have found thatgood results have been obtained in making the new fiufiy chromic oxideby rapid firing at about 23002500 F., and with limitation of the "asdivided into fourunit operations but it is not necessary that theoperations be so divided in actual manufacturing practice, since theymay be combined so as to reduce or increase the number of stages withoutdeparting from the invention. The four successive unit operations are asfollows: (1) Reduction of a solution of a chromate with an aqueousalkali sulfide emulsion of sulfur under regulated conditions to producehydrouschromic oxide; (2) Treatment of the hydrous oxide to remove sodacompounds; (3) Conversion of the hydrous oxide into a new sulfatecombination in the form of a syrup or soft friable coke; and (4) Firingunder regulated conditions to give the new fiufiy chomic oxide.

1. Fmsr STAGE-S LFUR Ranuorron As the first stage in the production ofthe new fiufly chromic oxide, a hydrous chromic oxide is prepared in aform such that it can be washed to eliminate sodium compounds. While theproduction of such a hydrous chromic oxide is not limited to theparticular method I have described, I find that a hydrous oxideparticularly adapted for use can be prepared by my new 4 The reactionproduces sodium thiosulfate and hydrous chromic oxide, proceedingessentially to theoretical completion in a manner which appears to beillustrated by the following type of reaction:

6N8.2Cld4+11S+34HzO- 3Cr2o3+ NaaSzOa.5HiO+NazS.9HaO

The following example will illustrate my new constant-alkalinity methodof making hydrous chromic oxide by maintaining substantially constantconcentrations of all soluble reacting and reaction products:

Dissolve 240 pounds of sodium sulfid (Na2S.9H2O) and 1240 pounds ofsodium thiosulfate (Na2SzOa.5 HaO) in 80 gallons of water near .theboiling point. Emulsify into this solution 1'76 pounds of powderedsulfur and heat until the boiling point is at some definite temperaturebetween 100 and 110 0., adding water if the boiling point should raiseabove the desired operation temperature, which by way of example may be105 C. Lowering the boiling point by dilution, somewhat improves thequality of the final product, but it increases the size of equipment andthe amount of water to be evaporated for recovery of thiosulfate. Nowrun slowly into the reaction kettle 486 pounds of sodium chromate(NaaClOc) dissolved in about 120 gallons of water. The rate of additionis such as to keep the boiling point constant. Nor should it be morerapid than the rate at which the chromate is completely re-- duced. Morewater maybe added either to the chromate solution or the reaction kettlein order to assist in the control of these conditions. For instance, ifthe boiling point goes above 105 'C.. it is an indication that morewater is required. The complete reduction of chromate at any stage maybe determined in any customary manner,

for instance, by transferring a drop to a piece of filter paper. Acharacteristic bright yellow color at the outer edges of the stainindicates unreduced chromate. The above reduction ma require from 15minutes to 2 hours.

Now emulsify into the reaction mixture an additional charge of 176pounds of powdered sulfur, followed by slow addition of another. lot of486 pounds of chromate dissolved in 120 gallons vof water, controllingrate and conditions as in the first case. Repeat with successive chargesof sulfur and chromate in exactly the same manner until the reactionkettle is filled to operating capacity, which may be, by way of example,a total of four charges. At the end of the last addition the kettlecontains essentially a suspension of crude hydrous chromic oxide in asolution of sodium thiosulfate together with some sodium Run an aboutthree-quarters of this mixture, and continue to charge the reaction in,as will be apparent to those skilled in the art. a

kettle .with successive amounts of sulfur and chromate solution,repeating all operations in the same manner as previously, and for anindefinite number of times. The green sludge in the liquor drawn offfrom the kettle is separated from the thiosuliate solution as bydecantation or filtration. It contains all the chromium in the form of ahydrous chromic oxide associated with some soda salts.

The aqueous solution first prepared may be made, as described, ofthiosulfate and sulfide, for example, in the molecular ratio of five toone; and the amount of water such that the boiling point of the solutioncorresponds to the reaction temperature, e. g., to C. The sulfuremulsified into this mixture with heat may go into solution as apolysulfide, such as ,NaaSs. All the sulfur does-not have to be added atthe start, but only sufficient, and with further addition from time totime, to keep the ratio somewhat above that to form NazSs. As it isconsumed by the chromate, more sulfur may be added.

The chromate solutionis added slowly with stirring and boiling, the ratebeing such that the boiling point remains constant. In this process theratio of thiosulfate to sulfide to water is practically the samenear'the end of the operation as at the start.

The method illustrated in the above example is adapted to continuous, orsemi-continuous, modifications, and variations may be made there- It isnot necessary to add thiosulfate at the start, as it is quickly formed,and the desired'equilibrium will be approximated by regulating theboiling point; but, if thiosulfate isnot added at the start, and if itis desired to establish the exact equilibrium, the originally addedsodium sulfide may be neutralized, since freshsulfide is generated inthe desired ratio simultaneously with the-thiosulfate formation. If theoriginal sulfide is neutralized in such a way as to simultaneouslyoxidize it to thiosulfate, only five-sixths'should be iieutralized toproduce the proper equilibrium rat 0. I

If a crude sodium chromate liquid is employed, it should first be freedfrom such impurities as lime and iron. Alumina and silica do notappearharmful. Reaction kettles should be used such as do not impart harmfulimpurities to the reaction mixture.

To explain further the reaction and the nature of the process, it may benoted that the hydrous oxide (CTZOILXHZO) is precipitated and is not insolution, and increasing amounts do not aiiect the equilibrium of thedissolved products. Likewise, the chromate is consumed as added, anddoes not change the equilibrium since the reaction products remaining insolution are in approximately the same proportions as those initiallypresent.

While I do not wish to limit myself to any theoretical explanation, itis my theory that under control of alkalinity.

I the use of thenew uniform type of hydrous oxide produced by the aboveprocess is advantageous.

2. Sscorm Srsor-Acm Tnssrmasr In preparing the new fiuffy chromic oxide,it is important to remove admixed and adhering alkali salts withsubstantial completeness: and it is important that the hydrous chromicoxide be I of such a character as to permit treatment and removal ofsuch alkali compounds. The hydrous chromic oxide complex produced inaccordance with the above process is well adapted for this purpose. r

Theraw green hydrous oxide sludge from the sulfur reduction is firstwashed with water to remove the bulk of the thiosulfate and othersoluble salts. This can be done to good advantage at 60 oi' tlmesuflicient to cause the hydrous chromic oxide to lose its ability tohold the alkali, to render possiblethe removal of alkali impurities.Digestion with dilute sulfuric acid has been found satisfactory, usingatemperature between 60-80 C... lmtilthe pH is down to about 3 and thenwashingto. remove acid and freed alkali impurities. Np harm results ifthe pH isa little lower or higher,

providing it remains fairly constant indicating that acid is no longerbeing consumed. The pH the hydrous chromic oxide, that is. during thedigestion period, should be maintained between approximately; and 5.when the pH of the acid liquor is above about 5 it does not act upon thehydrous chromic oxide to cause it to lose its ability tohold alkaliimpurities. If the pH. is permitted to go appreciably below 3,dissolution of th'e'hydrous chromic oxide in the liquor will take place.The above treatment removes alkali, as an alkali sulfate or bisuliate,down to at least 0.2%,

on the'basis of the Orzo:- The more complete I the soda removal thebetter the final product. Part of the hydrous chromic oxide may bconverted to an insoluble basic sulfate. At times a vary small amountmay go into solution, but seldom sunlcient to color the eiiiuent.Attrition at i this stage as for example in a ball mill tends to,

improve the product, out is not essential. Aluto 90 C. It is thentreated with acid for a length of the acid liquor during its digestivecontact with and violet' modifications are insoluble in ethyl alcohol.As commercially new chromium products they offer new fields ofapplication in various chemical industries, including textile andtanningindustries, etc. 3. Tman Srsos-Convsasrox r0 SYRUP The thirdstage of the process consists in combining the hydrouschromic oxide withan acid, for example, with B. sulfuric acid. While the proportions maybe somewhat varied, I have found the proportions most generallyeffective are Just above equivalent proportions to form the normalchromic sulfate. In the following example, an excess of about one-sixthis employed. Water enters into the reaction to eifect the desiredcoordination of the sulfate, which I am led to believe containsone-third of the 804 within the coordinated chromium complex. I haveformulated, by way of illustration, the type at reaction which I am ledto believe takes place zwnoterno vmso. ssmo (Cl'dBOOnlSHrOhHgSO. 201110(1630 YI'D") In carrying out the process, by way of example,

the hydrous oxide, sulfuric .acid, and water react in approximately theproportions illustrated by the above equation. iAt first the mixture ofhydrous oxide, sulfuric acid and water is a cloudy, green emulsion. Thetemperature of conversion may be from 110 C. to 140 0., depending on thetime of heating, proportions, character of hydrous oxide, manner ofstirring, etc. The chief reaction may occur at about 120 C. At about C.the color may become deeper green, and by C. much greener and clearer.This may be followed by some foaming and thickening, be-

coming greener and clearer. Between and C., it may start to skim overthe top, with some coking" at the bottom. At this stage the mine andmany acid radicals, as for instance S04 and P04, are beneficial.

conversion may be substantially complete. All or part of the 20H2Oindicated in. the above equa-.-

tion may be driven off during the conversion. If

the conversion is carried too far, it may be brought back by adding alittle water. with stir- As a result of this process there appears to beformed a new coordinated sulfate combination of the hydrous oxide. I amled to believe that the coordination formula of the green amp is of thefollowing type of single internal coordination, with varying amounts offree, or loosely held, sulfuric-acid and water:

[Cr2(H:O).io(SO4)](BO4)r Apparently the proper coordination of the sul-'fate combination is directly related to the struc- The. hydrous oxide,which has been prepared and purified as described in the foregoig, maybe employed as a wet sludgeor cake, or it may be dried undercontrolledconditions to produc hydrous oxides of various degrees ofhydration. The hydrous oxides prepared as above described are soluble inacids, for example,hydrochloric, sulfuric, phosphoric, acetic, oxalic,tartaric, etc. This aifords a method for improved manufacture of manychromium compounds more economically, and in greater purity and varietyof types, than has heretofore been practical by the usual methods. Forexample, it is possible to produce in this manner with ease many verywater soluble and \musal crystalline blue and violet chromic sulfatehydrates-as well as the amorphous varieties. 'Theblue 1 ture of thefinal fiuffy chromic oxide obtainable therefrom by regulated firing. I

/ 4. Fo'oam Brena-maro- The firing to produce the iiufly chromic oxidepigment can be carried out in successive stages,

by first heating the syrup to convert it into a coke," and by furtherhe'ating-of the coke under regulated conditions to produce the. desiredpig- I ment. The firing can, however. be advantageously carried out byadding the syrup directly. to the furnace. Both of these methods will bede-. scribed by way of example.

(I4) Conversion of swim .to coin: before firms On heating the syrup tozoo-25oo. it mg; duces a soft, friable "coke" having agreed-gray color.This product is reactive toward water, going quickly into solution, witha voluminous white colloidal separation, probably ofa basic chromicsulfate. On further heating, S: is driven oil, and the coke becomeswhiter in appearance. Analysis at this stage may corref spond toO:Cr.SO4.Cr:0.

On further heating, such as "fiash firing, the coke may yield thedesired flufiy chromic oxide. This firing is carried out by rapidlyheating the coke initially formed to a temperature of about 2400'F., andholding the material at this temperature for only. a limited period oftime, depending'somewhat upon the size of the furnace, the amount of.the charge, and the method of firing; but the product should beremovedfrom the furnace before the fiuify chromic oxide formed has beenoverheated, as this may convert it into a dense product.

(B) Syrup firin eous film. It is a ,difficult thing to observeaccurately, but it appears that a tough, resinous skin quickly fusesaround the entire charge. Then the mass swells to about 3 or 4 times itsliquid volume, expanding in all directions as the coke momentarily formsand sets, but without spattering or loss of material. Then small holesappear in the skin, and white fumes escape quickly, but without violenceor dust loss.

The time required depends on the tempera-v ture of furnace, and size ofcharge. It may approach flash firing. Ina small laboratory furnace at2400" F., the thermal decompositions may require one minute, withanextra two or three minutes to assure uniformity. Using lowertemperatures, or larger charges, which have slower heat transmission,the firing time may be increased to about 15 minutes, but leaving it inthe furnace much longer than this may tend towards the formation of theordinary dense chromic oxide.

In carrying out the firing, it is important, as hereinbefore pointedout, that the material be heated rapidly to the necessary hightemperature, without permitting it to come to equilibrium atintermediate stages; and it is also I important that overheating orprolonged heating be avoided such as would convert the new,

' intermediate fiufiy oxide into a final dense form.

The fluffy oxide produced by the present process does not stick to thehearth of the furnace but is readily removable therefrom. It may comeout much like a crumbly pancake, disintegrating on cooling to a veryfine, uniform, porous, soft, fiuify green powder. It requires nogrinding. It is the new intermediate flufiy chromic oxide having theempirical formula CraOs. It possessesthe superior oil absorption andother properties and applications hereinbefore referred to.

When the new product is made according to the foregoing illustratedprocedure, the appar 0.900; its oil absorption between 20 and 30; itstinting' strength unusually high; its mass color a superior, pure,bright green; andit is permanent in all respects. The following shows a'chemical analysis of such a type offluffy.

. green chromic oxide:

Percent CrzOs 99.15 A1203 0.18 S03 0.16 S102 0.12 FezOs l 0.07

The oil absorption and apparent specific gravity of the new fiuffychromic oxide green pigments are illustrated by the following table,showing some characteristic products produced by firing of the syrup,with indication of the firing temperature,- the oil absorption and theapparent specific gravity:

Apparent Firing am. r. m 8L assesses THEORETICAL EXPLANATION While I donot wish to limit myself by any theoretical explanation of thereactions, and of the reason for the formation of the new fiufiy chromicoxide, I have attempted to explain such formation by the followingtheory:

My theory is that the desired fluffy chromic oxide results fromthedirect decomposition of,

a single-internal coordinated chromium compound, and is harmed bygreater than one degree of internal coordination. Blue and violetmodifications of chromium compounds, which are believed to be solelyextemally coordinated, can be seen to shift instantly to an internalcoordination (green) when fired. The general idea is to eilect, completethermal decomposition to the desired oxide before the first degree toexclude furnace gases. A still further advanent specific gravity may bebetween0.300 andtage is the .intemal coke-like structure, in conJunction with the external skin, which sets and holds the mass rigidafter its initial swelling,

with the result that there is no opportunity for undesirablerearrangement, condensation, polymerization or crystallizationactionsbetween different molecularcomplexes. The formation of the desiredfluffy green oxide cannot be credited, however, entirely to the physicalcondition resulting from the coke and skin during the firing, because,when the syrup is made into a dough by kneading in an equal amount of anunconverted hydrous oxide, the' mixture, on firing, cokes and smnswitnout dusting, quite similar to the straight syrup, but the resultingchromic oxide only approximates, the average of the two oxides, iffiredv separately. In other words, the chromium compounds being fired,should have the correct coorexample, may be employed as dination, ormolecular structure, as well as desirable. outward physicalcharacteristics, in order to obtain the best results.

FLUFFY COMPOSITE Crmoruc Oxma PIGMENTS (A1203) on the basis of the finaloxide complex.

Practically all the S03 is driven off during the firing operation.

The presence of the alumina during the firing of the syrup or cokeappears to retard the speed at which the new chromic oxide changes tothe ordinary type at high firing temperatures, with the result thatthepresence of small amounts of ing different compounds manent charactercan thus be obtained by addto the chromic syrup and directly firing thesyrup-like, or potentially in- I tumescent compositions containing otherpigetc. Thus, for example,

harmful impurities, such as traces of soda, are

partially counteracted; and minor variations in operation, or rawmaterials, have'less effect on the shade of the finished product.

The alumina and chromic oxide appear to be in some form of combinationsince the properties of the new composite chrome-alumina pigment aredifierent from'those of-a physical mixture of the individual oxides inthe same proportions.

The resulting chrome-alumina oxide retains the new soft, porous, fiufiystructure and'is even a clearer, cleaner, brighter green color than thechromic oxide itself, notwithstanding the fact that less chromium ispresent.

JThe new chrome-alumina pigments possess the same general type ofdesirable properties and aP- plications as the pure fiufi'y greenchromic oxide, hereinbefore referred In). i

The apparent specific volume, oil absorption, and related properties areincreased by thefcombination with alumina so that they are somewhathigher than for the pure flufly green chromic oxide prepared undersimilar conditions. The

new composite pigments are exceptionally suitable for most purposes,including particularly use in printing inks and paints.

The followingchemical analyses illustrate two fiufiy chrome-aluminagreen pigments made by adding different proportions of aluminum sulfateto the chromic syrup before firing:

Per cent cent Orzo; -r 95.88 81.14 A120: 3.27 18.01 0.16-v 0.18

I In a similar manner it is possible to combine a .number of otheroxides with the new fiufly chromic oxide by stirring into the chromicsyrup appropriate compounds of other metals which are decomposable onfiring to give the corresponding oxide. Solutions of these other metalsalts may be added individually or together with salts of aluminum; andcomposite pigments containing two or more other oxides can be. similarlymade by adding decomposable compounds of these varitoils metals to thechromic syrup before firing; New and improved composite pigments oiaper- Per Cra I ment-formin'g oxides, to obtain composite pigmentscontaining oxides of iron, nickel, titanium, the chromic syrup, or thealumina-chromic syrup, may be combined and fired to produce many newpermanent pigments, by incorporation into the syrup, before firing,compounds such as those of cobalt, nickel, iron, manganese, tin, zinc,silica, titanium, etc. It is apparently possible to incorporate intopigments by this method almost any type of inorganic radical; althoughinorganic compounds such as sodium compounds should be excluded inParticularly good results have been obtained with combinationscontaining from 2 to 30% cobalt oxide (C00) on the basis of the finaloxide complex. Practically allthermally decomposable acidicradicals aredriven of! during the firing operation. The cobalt andchromium, or thecobalt, chromium and alumina, appear to be in combination since theproperties, including color, are distinctly different from those ofphysical mixtures ofthe pure oxides. For example, the strength andclarity of color of the new composite pigments are very much superior tophysical mixtures of pure cobalt blue and chrome oxide green in the sameproportions. In fact it is not possible to duplicate the new permanentblue-green pigments by mixture of known .permanent pigments.

The new combined cobalt-chrome pigments, and cobalt-chrome-a'luminapigments, possess the same general'type of desirable properties andapplications as the pure, fiufiy green chromic oxide. However, theyenable modified shades and colors to be produced, difierent from that ofthe pure chromic oxide. If a large proportion of metallic oxide, such asthat of cobalt, is present during firing, the furnace product may notdisintegrate to a powder on withdrawal from the firing furnace. It is,however, quite softand friable and readily breaks up or disintegrates toa fine, soft, porous, uniform, flufly powder on shaking or lightcrushing. 1 v The following chemical analyses illustrate two differentfiufly cobalt-containing pigments, made in the manner above described:

87.2 605 C00 12.0 21.3 A1203 -a----- None 16.9

Both of these pigments are exceedingly bright, clear, blue-greenpigments with exceptional tinting power, and a purity and clearness ofshade which cannot be duplicated by known permanent pigments so far as Iam aware. I

.The oil absorption and apparent specific gravity of the new compositepigments containing cobalt oxide, alumina, or both, isillustrated by thefol- Per cent Per cent cation,

lowing table showing the percent of the respec-' tiveoxides based on theiinal oxide complex:

Firing Oil Apparent temp. absorption sp. gr.

' paint vehicles, such as linseed oil, etc. The'new pigment compositionsthus made can be used for various purposes, such as those hereinbeforereferred to.

The new pigments can, of course, be used as tinting pigments, etc.,together with other pigments, in paint compositions, etc.

This application is a division of my application Serial No. 212,392,filed June "I, 1938, now Patent No. 2,209,899. Ido not, in thisappliclaim the new pigments, the new syrup form of a sulfate combinationof hydrous chromic oxide, the new coke-like sulfate combination ofhydrous chromic oxide, or the method oi' producing these productsdescribed herein, as this subject matter is claimed in said applicationSerial No. 212,392, In this applies-- tion I claimthe method ofproducing the hydrous chromic oxide described herein, the method oftreating hydrous chromic oxide with acid, as well as the uniform hydrouschromic oxide and the alkali-free hydrous chromic oxide describedherein.

I claim:

1. A uniform hydrous chromic oxide, obtained by reduction-of a sodiumchromate with an alkaline sulfur emulsion in water, under substantiallyuniform conditions of temperature and concentration of dissolvedreactants throughout the reduction, said hydrated oxide consistingsubstantially of a single type of hydrated hydrous chromic oxide.

2. The improved method or reducing alkali chromate and of producinghydrous chromic oxide which comprises gradually adding a solution of analkali chromate to a boiling alkaline emulsion of sulfur in a sodiumsulfide solution and regulating the alkalinity so as to maintain itsubstantially uniform throughout the process.

3. The method of reducing a sodium chromate and of producing a hydrouschromic oxide which comprises gradually adding a solution oi a sodiumchromate to a boiling emulsion of sulfur in a sodiumsulfide solution,and controlling the addition and the proportions oi! the reagents so asto maintain substantially uniform concentration oi dissolved reactantsthroughout the reaction.

4. The method of treating a hydrous chromic oxide resulting from-thereduction of an alkali chromate with an aqueous alkaline emulsion oisulphur which includes the step of digesting the hydrous chromic oxidewith a heated acid liquor. the pH oi which, during digestive contactwith the hydrous chromic oxide, is maintained be-.

tween approximately 3 and 5.

w 1 MARC DABRIN.

